The inventive concept relates generally to nonvolatile memory devices. More particularly, certain embodiments of the inventive concept relate to a nonvolatile memory device that performs read operations using a variable read voltage.
Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include static random access memory (SRAM) and dynamic random access memory (DRAM), and examples of nonvolatile memory devices include flash memory devices and read only memory (ROM).
Among nonvolatile memories, flash memory devices have gained increasing popularity in recent years due to attractive features such as relatively low cost, high storage capacity, low power consumption, fast access speed, and an ability to withstand physical shock. Flash memory devices can be broadly categorized as NOR type and a NAND type according to the layout of their memory cells and bit lines. A NOR type flash memory has a layout in which two or more cell transistors are connected to a single bit line. In the NOR type flash memory, data is stored by channel hot electron injection and erased by Fowler-Nordheim tunneling. A NAND type flash memory may have a layout in which two or more cell transistors are connected in series to a bit line. In the NOR type flash memory, data is stored and erased by Fowler-Nordheim tunneling.
Memory cells of a flash memory may store 1-bit data or multi-bit data, depending on the memory's design. Where a memory cell stores 1-bit data, it may have a threshold voltage corresponding to one of two threshold voltage states representing data “1” and data “0”. Where a memory cell stores 2-bit data, it may have a threshold voltage corresponding to one of four threshold voltage states representing data “11”, “10”, etc. Where a memory cell stores 3-bit data, it may have a threshold voltage corresponding to one of eighth threshold voltage states representing data “111”, “110”, etc.
In general, memory cells storing different numbers of bits may use different voltage levels to perform read operations. Moreover, a particular set of memory cells may be used alternatively to store 1-bit data or multi-bit data. Accordingly, it may be necessary to determine the read voltages to be used in different read operations according to the current configuration of a memory device.